After trekking across two mountains and wading through an icy river, the flat plains on the eastern bank of Iceberg might seem welcoming in comparison. However, you will still have to drop into a cave to reach Dark Matter's stronghold, and the labyrinth within can be confusing to navigate. Ice blocks are plenty and will block passageways in the cave, so be sure to find something to heat them up. If you know where you're going, you can perhaps help a snail find its shell, lost inside one of the many tunnels in the cave.
The frosty cave
Now that you've crossed the river, you're well on your way to King Dedede's castle. The starting path has many fragile ice bridges. Don't stand on them for too long, as they quickly collapse! Chillies and Klinko are common here, but you won't need their abilities for the Heart Star. Reach the end of the path and you will reach a door that takes you into a mountain cave. Here, watch out for Wappa that drop down from the holes in the ceiling. The slope of the ground makes it hard to attack them as they roll at you, and you will also have to contend with the Babut flying in the air. Luckily, there's a healing drink near the end of this section to replenish some lost health.
Burning up a path
You're at the top of a tall vertical space with multiple levels separated by disappearing ice blocks. There are a number of Galbos here, and you can easily get hit by their fire breath if the ice below you don't give way quickly enough. You do want to swallow one of the Galbos for the Burning ability, as it can melt through the ice blocks that are quite common in this stage. At the very bottom of this shaft are two ice columns for you to test out Burning's ice-melting ability. Burn them away to reveal doors underneath them, the left door leads to Coo and the right door leads to Rick. Coo is required for the Heart Star here, so he is the obvious choice. Go through the middle door to continue on.
The next few rooms are devoid of enemies, but full of obstacles. The first chamber contains many huge ice blocks, that are tricky to avoid, while the next room has tons of falling icicles. Patience and timing is the key to clearing these two rooms unharmed. After that, you must climb uphill in a section full of Corori, making sure to avoid the snowballs they push down. Then, it is a corridor full of Pacto on both sides. Finally, you will end up in a narrow hallway filled with Star Blocks. If you don't have the Burning ability, you will have to go through the first door you see. If you have the Burning ability, you can easily burn through the ice blocks beyond the door, which leads to several stars and health drinks. If you have both Coo and the Burning ability, you can dive into a small cave containing a door with two plant stalks, which is the path you need to take for the Heart Star. Otherwise, keep going left to reach a door that puts you back in the main path.
Heart Star: The snail and the yeti
A snail has lost its shell inside this snow cave, and you must find the shell to cheer up the gloomy gastropod. You must follow a set path to reach the snail's shell. If you've followed the walkthrough up to this point, you are now in a room with Chuchu and a door. Abandon Coo temporarily and take Chuchu, then use your Burning ability to inflate Chuchu, letting you ascend up the wall of blocks. Then, take Coo and use his fiery divebomb to remove the ice blocks in the next vertical shaft, which lets you reach Pitch. Finally, use Chuchu to rise up the final column of ice blocks. Go through the door at the top and fall down to find a ledge with the snail's shell. Touch it to collect it, earning you the Heart Star.
As you fall all the way down, you must now contend with a new mini-boss, Yuki. This pile of snow has a number of attacks, some of which can be a challenge to dodge. Its main attack is to throw three snowballs at you. These snowballs have various arcs and will land in different spots on the floor, even sticking to the ceilings, and they can block attacks aimed at Yuki. Yuki can also charge across the arena, knocking a snowball off of it when it bumps into the wall. Finally, Yuki can jump around the stage, mostly trying to land on you, launching blobs of ice on either side as it lands. You can inhale the snowballs created by any of Yuki's attacks and launch them back at it as a form of attack, if you lack a copy ability to directly harm it. Yuki will give you the Ice ability if you inhale its downed body.
Out of the ice cave
You've obtained the snail's shell and defeated a snow monster, but there are still a bit more to go before you reach the end of the level. You will end up in a corridor with Gabon tucked into small holes in the ground. Their arced bones are hard to evade, but can be easily neutralized by an attack. Next, you must pass through a hallway populated by Kabu and Dekabu. Concentrate on the Dekabu first, as the Kabu they spit out can overwhelm you with numbers. Past that room is the final section of the game, where you emerge from the tight spaces of the cave into the wide open ice plains. There are now Sasuke descending from the skies and Nruffs charging at you, so the danger is not over yet. Keep running along the flat path and you will eventually reach the exit door.